Bar code scanners generally operate by scanning a light beam across a symbol and translating the light reflected from the symbol into signals representing a bar code. The components of a bar code scanner include a light source to produce a light beam, an arrangement of mirrors and optics for light transmission and collection, a photo-detector to change returned light into electronic signals, and signal processing electronics to translate the signals produced by the photo-detector into timevariant signals for bar code decoding.
The strength or size of the photo-detector signals depends on the intensity of the returned light from the symbol. The returned light intensity varies with the distance of the symbol to the scanner, and the optical output power of the light source. The fluctuations in light intensity result in variations in the strength or size of the photo-detector signal, producing problems in signal processing. Specifically, the signal processing electronics contain a digitizer circuit for producing an analog bar code signal for bar code decoding. If the signal to the digitizer is too high, the digitizer becomes overloaded and cannot operate, while if the signal to the digitizer is too low, the digitizer cannot convert the signal with high accuracy.
This problem is typically addressed by providing amplification to increase the signal size prior to the digitizer. In the prior art, amplification has been provided by analog amplifier circuits which vary the amplification gain level using a feedback loop from a peak detector circuit. Also the prior art has incorporated digital potentiometers within the feedback amplifier circuit to digitally set the level of amplification gain. See U.S. Pat. No. 5,200,597 issued Apr. 6, 1993 to J. Eastman, et al. This results in a system which constantly searches for a particular amplification gain level during each scan of a symbol over many possible levels.
In addition to variations in intensity, the photodetector signals also vary due to the symbol being scanned, more particularly, to the contrast of the symbol from the symbol's background. It is desireable to provide amplification in a scanner which accounts for such variations in contrast, while controlling the amplification gain to increase signal size prior to the digitizer.
The present invention, instead of using many possible amplification gain levels, provides a means of amplification which requires only a few levels of amplification, preferably two. The system is calibrated according to each level of amplification to perform scanning within a desired distance range of operation. The system uses a laser diode as the light source within a laser diode circuit. This circuit may be accurately set by a processor to a particular laser optical output power level. The system calibrates the laser diode power output for each selected amplification gain level over the desired scanner operating range of distances. The laser power output level is set to the calibrated level for future scanning. During operation, the processor selects the amplifier gain level on each successive scan according to its programming. Each scan may be at a different gain level, or at a gain level determined responsive to the size of the input signal to the digitizer.